3GPP Long Term Evolution, LTE, is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project, 3GPP, to improve the Universal Mobile Telecommunication System, UMTS, standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. In a typical cellular radio system, wireless devices or terminals also known as mobile stations and/or user equipment units, UEs, communicate via a radio access network, RAN, to one or more core networks. The Universal Terrestrial Radio Access Network, UTRAN, is the radio access network of a UMTS and Evolved UTRAN, E-UTRAN, is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment, UE, is wirelessly connected to a Radio Base Station, RBS, commonly referred to as a NodeB, NB, in UMTS, and as an evolved NodeB, eNB or eNodeB, in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
In order to support high data rates, LTE allows for a system bandwidth of 20 MHz, or up to 100 MHz when carrier aggregation is employed. LTE is also able to operate in different frequency bands and can operate in at least Frequency Division Duplex, FDD and Time Division Duplex, TDD, modes.
The rapid evolution of wireless technologies has dramatically changed the way we communicate and interact with our environment. The next generation radio communication system, i.e., the fifth generation, 5G, takes it even one step further by connecting not only individuals but also all sorts of machines in real time. Especially, the expansion of machine-type communication, MTC, toward industrial applications is seen as one of the key differentiators of 5G.
The requirements on connectivity for industrial applications are very diverse and largely depend on the use case of an industrial application to be operated. Therefore, different Critical-MTC, C-MTC, i.e. ultra-reliable MTC, solutions will be needed under the same 5G umbrella network, as illustrated in FIG. 1. The dashed ellipse in FIG. 1 illustrates a subset of 5G design targets that an industrial application can take advantage of, including 10 to 100 times higher number of connected devices, about 5 times reduced end-to-end latency and a higher degree of availability with respect to today's wireless networks.
Besides the end-to-end latency, which refers to the time taken for a packet to be transmitted across a network from source to destination, the Critical-MTC concept should address the design trade-offs regarding transmission reliability, mobility, energy-efficiency, system capacity and deployment, and provide solutions for how to design a wireless network in a resource and energy efficient way while enabling ultra-reliable communication. As a consequence, to reach the design targets of C-MTC, it is essential to provide faster cell detection and thereby also faster cell access.
To detect a cell in LTE, a UE first needs to synchronize with the cell. To enable cell synchronization, the UE has to decode the Physical Cell Identity, PCI, and the radio frame timing of the cell. Then, the UE will be able to read the system information elements necessary for cell detection. To synchronize with a cell, the UE tunes its radio to a specific supported band or channel. Each cell has a primary synchronization signal, PSS, and a secondary synchronization signal, SSS, located at the center of each band. The UE then synchronizes with the cell by performing the following steps.
In a first step, the UE detects the primary synchronization signal, PSS, that is located in orthogonal frequency-division multiplexing, OFDM, symbol #6 of subframe #0, in each LTE frame. The PSS is repeated every 5th ms and is thus repeated in sub frame #5 (in the same OFDM symbol). Hence, after reading the PSS, the UE is synchronized with the cell on a 5 ms basis. With the help of PSS, the UE is able to obtain the physical layer identity, 0 to 2, using non-coherent detection.
In a second step, the UE finds the secondary synchronization signal, SSS, that is located in the same sub frames as the PSS, but in the OFDM symbols before the PSS. With the help of SSS, the UE is able to obtain the physical layer cell identity group number, 0 to 167, using coherent detection. When physical layer identity and cell identity group numbers are detected, the UE knows the exact PCI of the cell.
Hence, in LTE totally 504 PCIs are allowed and are divided into unique 168 cell layer identity groups where each group consist of three physical layer identities. As mentioned above, the UE detects the physical layer identity from the PSS and the physical layer cell identity group from the SSS. In one example, the physical layer identity is 2 and the cell identity group is 4. Then PCI=3*(Physical layer cell identity group)+physical layer identity=3*4+2=14. Once the UE knows the PCI, it also knows the location of the reference signals that are used in channel estimation, cell selection/reselection and handover procedures. Thus, the UE can proceed and access the cell.
Hence, in practice cell synchronization in the current LTE solution could be performed by decoding 2 OFDM symbols that are transmitted on a 5 ms basis, if a match is immediately found when trying to detect the PSS and SSS. However, due to e.g. bad signal properties this is not always possible and the actual detection time is generally much longer.
Thus, there is a need to provide continuous synchronization between UEs and base stations for reliable communication, in particular for mobile critical MTC wireless devices. There is also a need to reduce the time for cell detection and cell access to fulfil the requirements of low latency. Hence, there is a need for improved cell detection methods and arrangements.